A team of astronomers announced they have discovered the smallest and potentially most Earth-like extrasolar planet yet. Five times as massive as Earth, it orbits a relatively cool star at a distance that would provide earthly temperatures as well, signaling the possibility of liquid water.

"The separation between the planet and its star is just right for having liquid water at its surface," says astronomer and team spokesperson Stephane Udry of the Observatory of Geneva in Versoix, Switzerland. "That's why we are a bit excited."

But researchers do not yet know if the planet contains water, if it is truly rocky like Earth, which might make it hospitable to life as we know it, or whether it is blanketed by a thick atmosphere. "What we have," Udry says, "is the minimum mass of the planet and its separation" from its star.

The researchers say they detected the presence of two new extrasolar planets (exoplanets) around a red dwarf star, Gliese 581, 20.5 light-years away in the constellation Libra, based on slight motions of the star. Their discovery brings the total number of planets orbiting Gliese 581 to three; two years ago they made the initial finding of a planet there.

Udry says the group has submitted a paper for peer review and plans to publish a draft this week. "The claim is extremely interesting and the team is very credible," says astronomer David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., adding that he cannot judge the validity of the claim until the team publishes its data.

Researchers believe that many smallish exoplanets exist, but so far they have only found 13 "super Earths" weighing in at less than 20 Earth masses, compared with more than 200 heavier gaseous planets. Udry's group searches for smaller planets using a telescope called HARPS (High Accuracy Radial velocity Planet Searcher), which looks for stars that wobble slightly.

As a planet orbits, it pulls a star back and forth. The range of motion increases with the planet's mass, and the time needed for one orbit (or one back and forth) translates into its distance from the star.

The smaller of the new planets, dubbed Gliese 581 c, orbits at one fourteenth the distance between Earth and the sun. But the red dwarf is 50 times cooler than the sun. The group estimates that the planet would experience temperatures in the zero-to-40-degree-Celsius (32-to-104-Fahrenheit) range.

"It's sort of at the 'Goldilocks' distance," says Charbonneau—closer to its star and the heat would vaporize any water; farther away and water would freeze.

The big question is whether there really is water on Gliese 581 c's surface, which requires that its surface be solid. Udry says planets smaller than 15 Earth masses are likely to be rocky or icy.

Charbonneau is more cautious. A five-Earth-mass planet "sort of looks like Earth, but it sort of looks like Neptune. So which is it?" he says. "There's just no way to know."

Prior Water Claim Evaporated?

In related news, a recent report of water vapor on gaseous extrasolar planet HD 209458 b may have been premature. Earlier this month, an astronomer claimed as much based on measurements of starlight passing through the planet's atmosphere.

Charbonneau, who led the team that collected the data, says the analysis is unconvincing because the telescope itself may have introduced variation that could be mistaken for fluctuation in light coming from the star.

Observations by the Hubble or Spitzer space telescopes may resolve the question once and for all in coming months, Charbonneau says.